My conclusion is that it doesn't really matter. Since a gasoline engine was put on-oard to charge the battery, the Volt was never going to be a pure electric car. From the very beginning, it was thought to be a "series hybrid" (gas to electric to wheels), but now we see that it is really a "series/parallel hybrid" (gas to electric and gas to wheels).
So what?
If you drive less than about 40 miles per day, it will act more or less like an electric car, carrying around some extra weight in the form of a vestigial gas tank and small engine. If you always remember to plug it in, and never put a drop of gasoline in it, it should work fine*.
If I understand the design correctly, the difference in how the Volt operates and how a Prius operates isn't significant, except that one is tuned to rely more on its electric power path, and the other is tuned to rely more on its gasoline power path. It seems to me that GM could, with software changes, tune the Volt to work like a Prius. They could use the gasoline engine to send more torque to the wheels, and less to charge the huge battery.
*Actually, we need to ask GM... can the Volt run without a drop of gas in it, ever? Or will it annoy the user with warnings and chimes because it wants to run the gas engine from time to time to keep it fresh?